/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Equivalence;
import com.google.common.base.Function;
import com.google.common.collect.MapMaker.Dummy;
import com.google.common.collect.MapMakerInternalMap.InternalEntry;

import javax.annotation.CheckForNull;

/**
 * Contains static methods pertaining to instances of {@link Interner}.
 *
 * @author Kevin Bourrillion
 * @since 3.0
 */
@GwtIncompatible
@ElementTypesAreNonnullByDefault
public final class Interners
{
    private Interners()
    {
    }

    /**
     * Builder for {@link Interner} instances.
     *
     * @since 21.0
     */
    public static class InternerBuilder
    {
        private final MapMaker mapMaker = new MapMaker();
        private boolean strong = true;

        private InternerBuilder()
        {
        }

        /**
         * Instructs the {@link InternerBuilder} to build a strong interner.
         *
         * @see Interners#newStrongInterner()
         */
        public InternerBuilder strong()
        {
            this.strong = true;
            return this;
        }

        /**
         * Instructs the {@link InternerBuilder} to build a weak interner.
         *
         * @see Interners#newWeakInterner()
         */
        @GwtIncompatible("java.lang.ref.WeakReference")
        public InternerBuilder weak()
        {
            this.strong = false;
            return this;
        }

        /**
         * Sets the concurrency level that will be used by the to-be-built {@link Interner}.
         *
         * @see MapMaker#concurrencyLevel(int)
         */
        public InternerBuilder concurrencyLevel(int concurrencyLevel)
        {
            this.mapMaker.concurrencyLevel(concurrencyLevel);
            return this;
        }

        public <E> Interner<E> build()
        {
            if (!strong)
            {
                mapMaker.weakKeys();
            }
            return new InternerImpl<>(mapMaker);
        }
    }

    /**
     * Returns a fresh {@link InternerBuilder} instance.
     */
    public static InternerBuilder newBuilder()
    {
        return new InternerBuilder();
    }

    /**
     * Returns a new thread-safe interner which retains a strong reference to each instance it has
     * interned, thus preventing these instances from being garbage-collected. If this retention is
     * acceptable, this implementation may perform better than {@link #newWeakInterner}.
     */
    public static <E> Interner<E> newStrongInterner()
    {
        return newBuilder().strong().build();
    }

    /**
     * Returns a new thread-safe interner which retains a weak reference to each instance it has
     * interned, and so does not prevent these instances from being garbage-collected. This most
     * likely does not perform as well as {@link #newStrongInterner}, but is the best alternative when
     * the memory usage of that implementation is unacceptable.
     */
    @GwtIncompatible("java.lang.ref.WeakReference")
    public static <E> Interner<E> newWeakInterner()
    {
        return newBuilder().weak().build();
    }

    @VisibleForTesting
    static final class InternerImpl<E> implements Interner<E>
    {
        // MapMaker is our friend, we know about this type
        @VisibleForTesting
        final MapMakerInternalMap<E, Dummy, ?, ?> map;

        private InternerImpl(MapMaker mapMaker)
        {
            this.map =
                    MapMakerInternalMap.createWithDummyValues(mapMaker.keyEquivalence(Equivalence.equals()));
        }

        @Override
        public E intern(E sample)
        {
            while (true)
            {
                // trying to read the canonical...
                @SuppressWarnings("rawtypes") // using raw types to avoid a bug in our nullness checker :(
                InternalEntry entry = map.getEntry(sample);
                if (entry != null)
                {
                    Object canonical = entry.getKey();
                    if (canonical != null)
                    { // only matters if weak/soft keys are used
                        // The compiler would know this is safe if not for our use of raw types (see above).
                        @SuppressWarnings("unchecked")
                        E result = (E) canonical;
                        return result;
                    }
                }

                // didn't see it, trying to put it instead...
                Dummy sneaky = map.putIfAbsent(sample, Dummy.VALUE);
                if (sneaky == null)
                {
                    return sample;
                }
                else
                {
                    /* Someone beat us to it! Trying again...
                     *
                     * Technically this loop not guaranteed to terminate, so theoretically (extremely
                     * unlikely) this thread might starve, but even then, there is always going to be another
                     * thread doing progress here.
                     */
                }
            }
        }
    }

    /**
     * Returns a function that delegates to the {@link Interner#intern} method of the given interner.
     *
     * @since 8.0
     */
    public static <E> Function<E, E> asFunction(Interner<E> interner)
    {
        return new InternerFunction<>(checkNotNull(interner));
    }

    private static class InternerFunction<E> implements Function<E, E>
    {

        private final Interner<E> interner;

        public InternerFunction(Interner<E> interner)
        {
            this.interner = interner;
        }

        @Override
        public E apply(E input)
        {
            return interner.intern(input);
        }

        @Override
        public int hashCode()
        {
            return interner.hashCode();
        }

        @Override
        public boolean equals(@CheckForNull Object other)
        {
            if (other instanceof InternerFunction)
            {
                InternerFunction<?> that = (InternerFunction<?>) other;
                return interner.equals(that.interner);
            }

            return false;
        }
    }
}
